1. Field of the Invention
The present invention relates generally to motor driven pumps and compressors, and more particularly to submersible motor driven pumps and compressors having a torque transmitting assembly.
2. Description of the Related Art
The subsea industry is transitioning from being a new frontier where only large multi-national firms developing new drilling and completion technologies to explore and develop new hydrocarbon resources in thousands of meters of water and without existing infrastructure could participate to a more mature market with many participating companies utilizing hundreds of high specification drilling rigs, ever improving drilling and completion technologies and growing infrastructure.
With this maturity in the subsea market, new challenges are arising. Those challenges include maximizing production from maturing and marginal fields, lowering costs to be competitive with over resource plays such as shale oil in North America. Cost reductions have also become important with volatile commodity pricing. Costs saving programs being adopted by operators are seeking methods to reduce overall costs of subsea development by 30% or more. Included in these programs are challenges to product and service providers to provide lower cost solutions that are easier, simpler and quicker to implement and that reduce the need for many existing and high cost drilling, completion and production processes.
One area of transition in the subsea that is in need of new technical solutions to address the demands of the clients is in the area of subsea processing and pumps. Traditionally, much of the subsea production and processing activities occurred on topside platforms and production units connected to subsea christmas trees and manifolds through pipelines and other tubular products. This configuration requires large pumps and ancillary equipment to assist in the transportation of oil, natural gas and water to separation units, processors and injection and water disposal units. The need for these items of equipment contributes to higher costs and complexity, which in turn affects reliability and ultimate profitability.
The aging of the world's subsea fields has also created subsea pumping challenges as older fields and reservoirs begin producing greater levels of water and require increased pressure to produce. The use of seabed pumps has been shown to extend the life of a reservoir and improve field economics by helping maintain pressure through either the injection of water into the reservoir or directly boosting the flow from the reservoir. Maturing wells also provide greater challenges for pumping fluids consisting of higher proportions of gas to oil that are more difficult for traditional pumps to efficiently move.
Subsea production pumps generally fall into the following types:
Centrifugal: Helico-axial (Axial flow). These subsea pumps have been proven for large applications. These pumps are generally very large, have low efficiency and need high shaft speeds (up to 6500 rpm).
Centrifugal: Mixed flow. These pumps have been qualified for subsea applications. They generally provide higher efficiency and need lower shaft speeds (up to 5400 rpm).
Twin-screw: These pumps have on a few occasions been installed for seabed pumping applications and tested in downhole applications. They are generally highly efficient when handling high viscosity fluids, but have historically had low reliability, particularly in the presence of particles.
Electrical submersible pumps: These pumps are mostly of centrifugal type but can also be of positive displacement type and have generally been utilized for downhole applications and work well with high volumes. They have been used for selected injection applications.
Each of these types of pumps present certain benefits as well as detriments, including their ability to lift heavy oil, operate in deep water, handle high gas to waster fractions and ease of maintenance.
Each of the current pump solutions also has drawbacks due to their high power requirements and complex sealing designs for the deepwater. The high power requirements of the pumps impose a need for large electrical umbilical lines and variable speed drives to supply and manage the needed power. Similarly, required operating water depths have stretched the pressure sealing capabilities of the equipment by their reliance on sensitive high pressure mechanical seals and associated complex barrier fluid systems for lubrication.
In recent years, technological advances have enabled greater use of subsea pumps and processing. These systems, however, still require expensive and large topside equipment to operate and cannot be economically used for smaller or marginal field developments such as “brownfields” or smaller “green fields”. In addition, larger and more complex equipment create challenges in enabling operators to engage in early field production.
There is therefore a need for a high performing and economical subsea pump system with the following characteristics: (i) is deployed subsea and can be operated without topside hydraulic pressure controls and large separate variable speed drive systems, (ii) is designed primarily for smaller field developments and flow requirements with motor power requirements of less than 1.5 megawatts, (iii) is seal-less so as to eliminate internal fluid leakage to the environment through dynamic seals, and (iv) is flexible and modular so as to allow for its incorporation in a large variety of applications, including boosting, seawater injection, water separation and fluid transport. A desirable system would also be capable of handling multiple types of fluids and fluid phases.
A subsea pump with the above characteristics could become a key component in systems that would enable:                Brownfield development of mature fields;        Development of greenfields with low initial pressures;        Injection of separated water from production fields;        Early production of discovered hydrocarbons;        Injection of raw seawater;        Subsea storage;        Deep heavy oil production;        Long-tie backs and flow assurance; or        Gas compression and seawater dewpointing/dehydration.        
Auxiliary applications, crucial to well-functioning subsea factory concepts being pursued by many oil and gas operators, include:                Active cooling pump using seawater or coolant in a loop to control temperatures of flows to and from the well, pipelines (e.g. “cold flow” technology) or equipment;        Condensate pumping to host/shore in relation to subsea gas wells;        Re-injection of oil into the flow to host/surface, post subsea separation systems;        Injecting condensate to stabilize wet-gas compressors; and        Wet-gas boosting.        